Electronic converters for light sources comprising, for example, at least one LED (Light Emitting Diode) usually supply an output direct current. Such a current may be stable or vary in time, for example, in order to regulate the intensity of the light emitted by the light source (so called “dimming” function).
FIG. 1 shows a possible lighting system comprising an electronic converter 10 and a LED module 20, including at least one LED L.
Electronic converter 10 usually comprises a control circuit 102 (for example a microprocessor) and a power circuit 104 (for example a switching power supply AC/DC or DC/DC) receiving as input a supply signal (for example from the electric line) and outputting, through a power output 106, a direct current. This current may be stable or can vary in time. For example, control circuit 102 may set, via a reference channel IRef of power circuit 104, the current required by LED module 20.
For example, LED module 20 may also comprise an identification element, which identifies the current required by LED module 20 (or in general control parameters). In this case, control circuit 102 communicates with the identification element, and adjusts the operation of the electronic converter to the operating conditions required by the LED module.
FIG. 1 also shows two switches 108 and 110. The first switch 108 permits to regulate the light intensity emitted by LED module 20. For example, switch 108 may be driven via a pulse-width modulation (PWM) in order to selectively short-circuit LED module 20, while deviating the current from generator 104 onto switch 108. However, a brightness regulation of the light intensity emitted by LED module 20 can also be achieved by setting a lower reference current IRef.
The second switch 110 allows deactivation of the supply to LED module 20. For example, an electronic converter may deactivate the supply when an error condition is detected, or for reasons of reliability, for example when an over-current, over-voltage or over-temperature condition is detected.
Generally speaking, LED module 20 may comprise one single LED or a LED chain or string, wherein a plurality of LEDs are connected in series. However, most international safety standards, such as for example the IEC (International Electrotechnical Commission) or the UL (Underwriters Laboratories) standards, classify LED modules according to electric or fire risk ratings, on the basis of their maximum supply voltage. As a consequence, a higher supply voltage leads to a lower safety rating, which can also raise the safety requirements of the mechanical structure of the LED module (and therefore the cost thereof) and can set constraints to the final applicability of the product.
As a consequence, in order to reduce the maximum supply voltage, LEDs may also be divided into plural branches connected in parallel. For example, as shown in FIG. 1, each LED module may comprise a plurality of LED strings connected in parallel, or power supply 10 may feed a plurality of LED modules 20 connected in parallel.
However, as a consequence of manufacturing tolerances, each LED may have different electrical and optical features, possibly causing a lack of uniformity in the light created by LED strings.
In order to avoid such problems, lighting systems have been known wherein each LED string comprises a dedicated current regulator, such as for example a linear current regulator, and LED modules 20 are fed with a voltage, i.e. electronic converter 10 may create on line 106 a fixed voltage. Such a voltage may also be set on the minimum possible value which is determined, for example via sensors detecting the voltage drop across the linear regulators.